This application claims the priority of German patent document 196 40 280.8, the disclosure of which is expressly incorporated by reference herein.
This invention relates to a toolhead for the precision cutting of a surface of a workpiece, and a method of manufacturing the same.
A toolhead of this type is known from German patent document DE 44 01 496. The toolhead is used in the precision machining of internal and/or external geometrical shapes of a workpiece with the toolhead or the workpiece rotating during the machining. The toolhead has an elastically movable joint in the form of a solid body, around which a tool bar connected thereto is pivotable. The tool bar is provided at the free end with a cutting tool for cutting the workpiece.
The solid joint is U-shaped in cross section. One leg of the solid joint is connected to a tool spindle or a tool holder, the other leg with the tool bar and both legs are connected by a web through which the pivot axis of the solid joint passes. The leg connected with the tool bar is elastically pivotable, around the pivot axis, in relation to the leg connected with the tool spindle or the tool holder.
The leg of the U-shaped solid joint which is connected with the toolbar is subject to the action of a piezo transducer which exerts on that leg of the solid joint which is connected with the tool bar a higher or lower force according to the applied electric voltage. The piezo transducer thus generates the control force required for the adjustment from an initial position to a cutting machining position.
The U-shaped solid joint at the same time forms a restoring spring which, on the reduction of the pressure generated by the piezo transducer, pivots back into a starting position, causing the tool bar with the cutting tool to follow.
This construction provides an extremely accurate method of cutting different shapes by means of a rotating or stationary tool, such as circular as well as non-circular geometrical shaped workpieces having any desired external profile contour.
Non-circular and also non-cylindrical workpiece geometry calls for deflections of the cutting tool with different control distances over 360.degree. of a workpiece or toolhead rotation. Piezo transducers, however, can only be subjected to compressive force, as the quartz or the connection with the necessary mechanical force transfer element allows only slight tensile forces to be imparted.
With the known type of toolhead this drawback is overcome by causing the piezo transducer to subject the solid joint only to compressive forces which enables one leg to be elastically pivoted in relation to the other. As the maximum deformation is well within the elasticity range of the material of the solid joint, such as steel, the joint is automatically restored to the initial position, as previously referred to, when the compressive force generated by the piezo transducer is removed or reduced. In order to ensure that no undefined deformation state can occur a suitable prestressing is adopted.
A toolhead of this kind suffers from the drawback that the behavior of a piezo transducer can be compared to that of an electrical capacitor and this then sets limits to the sphere of applicability and increases the control and regulating cost. Furthermore, high operating voltages are involved, so that transformers are required.
German patent document DE 32 45 053 makes known a machine tool with an adjusting device for a cutting tool which is mounted on a tool bar which consists of two elongated interconnected members of different magnetostrictive materials and which is surrounded by an electric coil generating a magnetic field. The tool bar serves as an actuating drive for pivoting the cutting tool. The magnetostrictive change in length produced by the two members of the tool bar on application of a magnetic held, for example the expansion of one and shrinkage of the other, or differences in the respective expansions or shrinkages of the two members, cause the aforementioned deflection of the cutting tool. The tool bar with the cutting tool is bent by the magnetic field and thus directly deflected.
This construction can produce a hysteresis resulting from two materials operating in opposition to each other and having different expansion coefficients. Furthermore, the production of the combination of materials with the use of adhesives or by welding is an expensive process.
An object of this invention is to provide a toolhead which will be simpler to use and in which the aforementioned drawbacks are avoided. In particular, the toolhead can be easily adaptable to existing frame conditions.
This and other goals have been met according to the present invention by providing a toolhead for precision cutting of a surface of a workpiece during a rotation of the toolhead or of the workpiece, the toolhead incorporating a cutting tool which is movable against a restoring spring force from an initial position to an adjusted position with respect to the workpiece by means of an actuating drive which comprises a translatory movable driving element and a transfer element which converts the driving movement into an adjusting movement, wherein the driving element for the adjusting movement of the cutting tool from the initial position into an adjusted position comprises an axially positively magnetostrictive expansion bar which is made in one piece and which is associated with an adjustable force transmission element by means of which the initial position of the cutting tool and the prestressing or preloading of the restoring spring force are continuously adjustable.
The above-stated and other goals have also been met according to the present invention by providing a toolhead, comprising: a base portion; a driven element coupled to the base portion, said driven element being movable relative to the base portion and being biased by a restoring spring force toward said base portion; a magnetostrictive expansion bar fixedly coupled to said base portion, said expansion bar being selectively engageable with said driven element to move said driven element away from said base portion.
This and other goals have also been met according to the present invention by providing a method of manufacturing a toolhead, comprising the steps of: forming a base portion; coupling a driven element to the base portion such that said driven element is movable relative to the base portion and is biased by a restoring spring force toward said base portion; and fixedly coupling a magnetostrictive expansion bar to said base portion such that said expansion bar is selectively engageable with said driven element to move said driven element away from said base portion.
This invention is based on the realization that ferromagnetic bodies undergo reversible changes in length through magnetic reversal and that this effect can be utilized to produce an adjusting force for a cutting tool. The magnetostriction can be positive--extension in the magnetization direction, for example using iron--or negative--shortening in the magnetization direction, for example using nickel. By the use of the magnetostriction effect for the actuating drive of a toolhead it is rendered easily adaptable to standard operating voltages (5v, 24v, 220v . . . ), thus also enabling a simple electrical control system to be used of low cost, for example by the switching current using low voltage transistors.
High adjusting frequencies are also obtainable using the electric magnetic field of the magnetostrictive actuating drive, providing a wide range of shapes using a simple control system.
According to the choice of material for the extension bar, relatively large length changes and adjusting forces are obtainable. The adjusting movement of the cutting tool can thus be adopted in a simple manner to requirements arising in practice.
By means of an adjustable force transmission element associated with the expansion bar, the initial position of the cutting tool and the prestressing of the restoring spring can be continuously adjusted, so that not only the feed movement but also the return movement of the cutting tool can be optimized. Any desired contour shapes can now be reproduced in a simple manner. Continuous adjustment is necessary for adaptation to different conditions of use, different operating temperatures and the desired behavior of the restoring spring.
In this system the drive for the adjusting movement of the cutting chisel from the initial position into a machining position is generated by axial positive magnetostriction.
The drive for the adjusting movement from a machining position in the direction of the starting position is generated by cancelling the axial positive magnetostriction and also by the restoring spring. This ensures that the adjusting movement from a machining position towards the starting position can mainly be effected without any time lag.
The continuous adjustment of the cutting tool not only enables any desired contours to be produced but also enables the cutting tool to be readjusted during the cutting process, without any breaks or flaws in the surface shape to be obtained. Very tiny adjustment steps are rendered possible with maximum frequencies.
In order, in particular, to ensure accurate machining of the workpiece, the adjusting movement of the cutting tool is controlled. For this purpose a measuring element is provided, serving to detect the adjusting movement of the cutting tool and forming part of a control and/or regulating device for adjusting the position of the cutting tool.
For the production of certain contours and shapes, the cutting tool performs at least two different adjusting movements per rotation, adjusting speeds of 1 to 10 kHz being obtainable. The adjusting movement distances are in the range 0-200 .mu.m. The regulating distance can be increased with the use of transfer elements. The adjusting movement of the cutting tool preferably takes place about an axis perpendicular to the longitudinal axis of the tool bar.
In a toolhead according to this invention this object can be easily attained with a restoring spring formed by a machined metal part subdivided by at least one horizontal recess into a base and a driven element, these latter being interconnected by a web. In particular, the web concerned is arranged eccentrically, so that the driven element, in the event of positive magnetostriction, is pivoted with respect to the base around a swivel axis situated eccentrically with respect to the rotation axis and passing through the web.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.